Method for removal of hydrogen halide from a process stream



Aug. 3, 1965 R. w. HAlsTY ETAL METHOD FOR REMOVAL OF HYDROGEN HALIDE FROM A PROCESS STREAM Original Filed Aug. 6, 1959 Il o United States Patent O 3,197,942 METHOD FOR REMOVAL F HYDRGGEN HALTDE FROM A PROCESS STREAM Robert W. Hoists', Richardson, Tex., and .lohn W. Ross,

Cumberland, RJ., assiguors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Continuation of application Ser. No. 831,966, Aug. 6, 1959. This application Apr. 7, 1964, Ser. No. 359,545

9 Claims. (Cl. 55-71) This application is a continuation of our earlier filed copending application Serial No. 831,966, liled August 6, 1959, -now abandoned, and entitled Method for Removal of Hydrogen Halide From a Process Stream.

rDhis invention relates to the selective adsorption of hydrogen halide from a process stre-am and more particularly to a method for removal of hydrogen chloride `from a process exhaust stream of a hydrogen reduction reaction to enable recycling the unused hydrogen and other reactants for reuse in a continuous reduction process.

The invention ywill be `described with particular reference to the reduction of trichlorosi-lane to high purity silicon suitable yfor use in lproducing transistors, but it should be understood that the invention is not limited to suc-h use.

The reduction reaction of trichlorosilane with hydrogen takes place in a quartz tube at a temperature of about l250 C. rBhe process stream as exhausted from the quartz tube includes unreacted hydrogen, trichlorosilane, and hydrogen chloride. Removal ot hydrogen chloride is necessary betere recycling the unreacted components plus make-up gas.

Accordingly, it is a primary object of the invention to provide a method of selectively removing hydrogen chloride from a process stream and especially -a process exhaust stream taken from a hydrogen reduction reaction to enable the process stream to be recycled in a continuous, eihcient and less costly manner.

Another object of the invention is to provide a method of removing hydrogen chloride from a gas stream which utilizes an eilicient adsorbent stable to attack by hydrogen chloride and which may be easily regenerated or reconstituted by desorption, thereby .permitting repeated adsorption-desorption cycles and efc-ient reuse of the 'adsorbent.

A still further object of the invention is to provide a method of removing hydro-gen chloride from a gas stream which utilizes an adsorbent of large capacity and capable of regeneration for reuse by simply heatingunder vacuum. p

Yet yanother object of the invention is to provide a method of removing hydrogen chloride from a gas stream which utilizes an adsorbent capable of regeneration for reuse by heating and purging with a nonreactive gas (inert with respect to the adsorbent). Examples of a suitable gas would include hydrogen, nitrogen, and air. These are the most economically feasible. Others, less practical from a cost standpoint, would include helium, neon, argon and in fact any non-reactive g-as.

The Inovel teatures that are considered characteristic of the invention are set :forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of the best mode for carrying out the invention and specie embodiments of the same when read in connection with the accompanying drawings,`wherein like reference characters indicate like .parts throughout the several iigures and in which:

FIGURE 1 is a chart illustrating the effect upon the 3,197,942 Fatented Aug. 3, 1965 ,a ICE adsorptive capacity of heating synthetic zeolite molecular sieves, both modified and unmodified, in an atmosphere of hydro gen chloride FIGURE 2 is a chart illustrating the stat-ic adsorption of HC-l by modified and unmodified Type 5A Linde Molecular ,Sieves under condi-tions of repeated adsorptiondesorption cycles; and

FIGURE 3 is a chart similar to FIGURE 2 but illustrating the effect on adsorption capacity of modifying Type 4A Linde Molecular Sieves.

Again referring to the previously described production of pure silicon by hydrogen reduction of trichlorosilane, the method of the present -inventioncomprises the insertion of a suitable adsorbent in the path of the exhaust gases issuing from the quartz tube reactor to remove selectively the hydrogen chloride from unused hydrogen and trichlorosilane. These unused reactants are then fed to a second quartz tube reactor, or recycled into the rst reactor, with or Without make-up gas. The silicon obtained in the reaction is suilic'iently pure ior semiconductor devices or can be lfurther refined by any suitable technique as by zone refining. A continuous production process is maintained by periodically removing the adsorbent material with substitution of fresh adsorbent, regenerating the saturated material by hea-ting in a vacuum or otherwise, and purging with a nonreactive gas and reinserting the regenerated adsorbent in the exhaust stream of the reactor.

A suitable adsorbent for use in the described process was extremely diiiicnlt to discover. The common adsorbents la'ck adequate adsorptive capacity at room temperature, are unstable at high temperatures, are subject to modifica-tion by HCl, or cannot be regenerated easily or frequently for reuse. These diiculties, to a certain extent, are experienced also by a relatively new synthetic zeolite, Type A, marketed by the Linde Air VProducts Co., under the name Linde Molecular Sieve, Types 4A and 5A. The new zeolite isidentiiied as zeolite A, and is described in det-ail in U.S. Patent No. 2,882,243, issued April 14, 1959. rl'lle structure of the zeolite is a threedimensional network of alternating A104 and SiO., tetrahedra, l2 of each per unit cell. Interstices are occupied by 12 sodium ions and 27 molecules of Water. The crystal unit cel-l provides a large central cavity 11.4 A. in diameter which is connected to six like cavities by restricted openings 4.2 A. in diameter. In additon, -a second po-re system lis provided by 11.4 A. cavities alternating with 6.6 A. cavities separated by 2.0 A. restrictions. While this zeolite structure is suitable for adsorption of hydrogen chloride and exclusion of trichlorosi-lane, it is not stable to attack by the `HC1 at high temperature. Hot concenrtated hydrochloric acid dissolves the zeolite in 20 to 30 minutes, with the formation of 4a gel. However, it was found that the Linde Molecular Sieves may be modied by ion exchange using zinc, calcium, and magnesium compounds to yield adsorbents entirely satisfactory for use in this invention.

As an example, modification of the Linde Molecular Sieve, lType 5A, was accomplished -by soaking the sieve in a 12/2% .aqueous 'solution of zinc chloride at room temperature for 24 hours. The product formed was much more stable to attack by hydrogen chloride, both aqueous and anhydrous, than the origin-al sieve. The zinc chloride treated sieve was heated for a period of up to two days in concentrated hydrochloric acid, during which time only a slight amount of dissolution land erosion occurred in contrast to the complete dissolution of the original material in 30 minutes. The zeolite pellets remained intact and, after washing in distilled water until it tested negative for chloride ion and drying at 420 C., the pellets had a capacity for adsorbing 4.5 grams of HCl per grams molecular sieve. Results with the zinc .9 chloride-treated type 4A Linde Molecular Sieves `were similar.

Referring now to the drawings, FIGURE 1 illustrates the etiect of subjecting modified (solid line) and unmodiiicd (broken line) Type A Molecular Sieves to alternate adsorption and desorption of hydrogen chloride. The first three cycles were carried out Iby s-aturating the sieves with HC1 at room temperature and regenerating under -a vac- -uum -at 300 C. Points 114 and 16 on FIGURE 1 refer to a change in procedure, At points 14 and 16, the sieves Iwere saturated with HC1 at room temperature, evacuated at room temperature, sealed olf and heated to 300 C. for 2O minutes. The HC1 pressure rose to about 580 mm. of'Hg. Subsequent desorption processes were carried out in the original manner by purging with hydrogen followed by evacuation. It is apparent from thel chart that the Linde Molecular Sieve Type 5A, curve .10, at iirst rises in HC1 .adsorptive capacity, but then begins to decrease in capacity and after contact With HC1 at 300 C., loses essentially all of its adsorptive capacity. The zinc chloride treated sieve, curve 12, however, While decreasing somewhat in adsorptive capacity after s-ix regenerative `cycles retains'a capacity in excess `of 4 grams HC1 per 100 grams molecular sieve and the adsorption capacity was not appreciably aiiected by the 300 C. contact with HC1. l

The modified form is also stable at higher temperatures than the original Type 5A Molecular Sieve which, after having lbeen heated to 800 C., had a capacity for adsorbing HC1 of only 1.48 grams per 100 grams molecular sieve. The Zinc chloride treated molecular sieve (5A) was testedfor eight.l HC1 adsorption-desorption cycles in wvhich the desorption was done at 800 C. The maximum capacity found was `8.11 grams HC1 per 100 grams molecular sieve, and the capacity after eight cycles Was 5.13 grams HC1 per 100 grams molecular sieve.

FIGURES 2 and'3 illustrate the static adsorption of hydrogen -chl-oride on modified Linde Molecular Sieves, Types 5A and 4A, respectively, after many repeated adsorption-desorption cycles. In each case, the hydrogen chloride pressure was 60 cm. of Hg and the adsorption temperature Was maintained at 30 C. Desorption in each cycle Was by evacuation at 420 C. Curve 20 illustrates that the Type 5A Linde Molecular Sieve loses its adsorptive capacity quickly as compared to a zinc chloride treated sieve, curve 22, which retains high adsorptive capacity even after thirty repeated regenerative cycles. The capacity of the Type 5A sieve is considenably increased by a soaking treatment with an aqueous calcium chloride solution (followed :by Washing with H2O); see curve 24. Similar results are obtained in the repetitive adsorptiondesorption cycling of zinc and calcium exchanged Type 4A Linde Molecular Sieves, see curves 24 and 28, respectively, of FIGURE 3.

The described properties of Zinc land calcium exchanged Linde Molecular Sieves, including resistance to attack by hydrochloric acid, good selectivity for hydrogen halides, lstability at high temperature an-d retention of adsorptive capacity after many repeated regenerative cycles make these modified sieves admirably suited for use as an adsorbent `in the method of the instant invention. This is particularly so because of the simplicity of the modification which involves merely soaking a sieve in an aqueous Zinc, calcium, or magnesium salt solution followed by washing in H2O'. The modified sieves are then inserted in the exhaust gas stream of the hydrogen reduction reactor to efficiently adsorb the hydrogen chloride without iadsorbing the unused hydrogen and metallic chloride which are recycled to the reactor or to a second reactor in the continuous reduction process. When the adsorbent sieves are saturated with hydrogen chloride, fresh sieves are substituted, and the used sieves regenerated by heating under vacuum, heating while purging with an inert gas, or by =a combination of these oper-ations. After regeneration, the sieves are, of course, available for reuse in the reduction process by reinsertion in the exhaust gas stream.

Table I shown below illustrates the eiiect of varying CaCl2 concentration and soaking time on HC1 adsorbing capacity of Linde Type 4A Molecular Sieve.

Table I [Effect of CaClz concentration and soaking time on HC1 adsorbing property of Linde Type 4A Molecular Sieve] Concentration of CaC12, Wt. percent 1 Soaking time, hours Capacity for HG1 after 9 cycles, wt. percent essa/ samenwer@ @rassegne 909052509090 1 Volume of CaCl solution used is 50 grams CaClz per 10 grains Molecular Sieve at each concentration.

2 Lost sample.

Table II illustrates similar data wit-h yregard to ZnClz.

` Table Il [Effect of ZnClg concentration and soaking time on HC1 adsorbing property of Linde Typef5A Molecular Sieve] Concentration of ZnClz, Wt. percent 1 Soaking time,

Capacity for hours HC1 after 9 cycles, wt. percent 1 Volume of ZnCl2 solution used is 50 grams ZnClz per 10 grams Molecular Sieve at each concentration.

The process of the invention, as described in detail above, was used in the production of silicon. The element was derived from trichlorosilane in a reduction reaction with hydrogen using a quartz tube as a reactor. Elemental silicon deposited on the quartz tube and H2, HC1, and unused trichlorosilane exited from the tube. A ZnCl2 modiiied Linde Type 5A Molecular Sieve was interposed in the spent process stream exiting from the quartz tube and served to adsorb HC1 out of the spent process stream. The gas stream issuing from the sieve,

plus make-up was recycled tothe quartz tube.

The sieve (55 pounds) was contained in an 8 foot, Type 316 stainless steel column of 65/s inch diameter. This quantity of sieve is sufficient for a four-hour run, based on an adsorption capacity of 6% HC1 by weight.

Four actual runs were made. N-type silicon was obtained in each case. Resistivity of the silicon obtained was about ohm centimeters. 1

There now follows a summary of dynamic stability tests performed on a calcium chloride treated Linde Type 4A Molecular Sieve. t

Preparation of sieve: 750 grams of Linde Type 4A Molecular Sieve were soaked for 108 hours (4l/z days) in an aqueous solution of 10% calcium chloride by weight. The sieve was then washed until the washings were chloride free, and ,dried at 420 C. under vacuum in a 2 inch diameter stainless steel column in which the adsorption runs were also made.

Initial HCl adsorption-desorption cycles: The iirst 24 cycles were made with adsorption at' room temperature and a pressure in the column of about one p.s.i.g. The initial capacity for HC1 (9.7% by weight) dropped to an average of about 8.5% for the first 24 cycles. Desorption was carried out by heating to 420 C. in a hydrogen purge stream. The 25th adsorption was carried out at a pressure of about 16 p.s.i.g. in the column (the gas stream used in all cases was 4% (mol) HCl-96% (mol) H2) to test the stability of the calcium chloride treated molecular sieve ata higher pressure.

Results of pressure test: The next 19 adsorption cycles, after the run at high pressure, showed an average capacity of 7.5%, which is about what follows from extrapolating the decrease in capacity with use at low pressure; thus, the material appeared to be stable at least up to the pressure of about 16 p.s.i.g. Desorption was conducted as noted previously.

Stability at high adsorption temperature: Beginning with adsorption number 45 (45th cycle) the sieve was subjected to adsorption cycles at ambient temperatures between 50 C. and 350 C., each high temperature adsorption being followed by the usual regeneration (desorption) at 420 C. in a hydrogen stream, and then an adsorption at room temperature, followed by regeneration, before the next high temperature adsorption. The results, summarized in Table III below, show that the treated molecular sieve is stable up to at least 350 C. in hydrogen chloride.

It is also apparent thatthis treated molecular sieve has sufficient capacity for practical application at ambient temperatures to at leastV 250 C.

Table III [Efeet of high temperature on stability and capacity for HC1 adsorption on calcium chloride treated molecular sieve] Adsorption Temperature HC1 adsorbed, number during wt. percent adsorption 44 Room 7. 26 45 50 C. 7. 55 46 Room 6. 91 47 100 C. 6. 18 48 Room 7. 42 49 150 C. 5. 29 50 Room 7. 25 51 200 C. 3.08 52 Room 6. 80 53 250 C. 2. 75 54 Room 7. 73 55 300 C. 2.02 56 Room 6. 51 57 350 C. 1. 72 58 Room 6. 66 59 Room 6.81 60 Room 7.19 61 Room 6. 93 62 Room 7. 0U

6 Regeneration in an air stream: Beginning with adsorption number 69 (69th cycle) the regeneration (desorption) was done at 420 C. in a stream of dry air instead of hydrogen. The results, summarized in Table 1V below, show that dry air is a suitable purge gas.

Table 1V [Regeneration with dry air (20 1iterlmin.)]

Adsorption Regeneration HC1 adsorbed, number purge gas wt. percent 65 Hydrogen 6. 28 66 Hydrogen 6. 62 67 Hydrogen 6.60 68 Hydrogen 6. 34 69 Dry air 6. 19 70 Dry air 5. 92 71 Dry air 6.19 72 Dry air 5.82 73 Dry air 5. 80 711 Dry air 5.80 75 Dry air 5. 80 76 Dry air 5. 80 77 Dry air 5. 80 78 Dry air 5. 80 79 Dry all' 5. 80 80 Dry air 5. 80 81 Dry air 5.80 S2 Dry air 5. 80 83 Dry air 5. 80 84 Dry air 5. 80

Y 85 Dry air 5. 80 86 Dry air 5. 80 87 Dry air 5. 80 88 Dry air 5.80 89 Dry air 5.80 90 Dry air 5. 80 91 Dry air 5.80 92 Dry air 5. 80 93 Dry air 4. S0

In summary, one and the same sample of molecular sieve, treated in the manner described above, with calcium chloride, withstood 93 adsorption-desorption cycles of HC1 under various adverse conditions. The experiment was discontinued after 93 cycles, since this was considered suiiicient proof of stability.

In addition to the improvements noted above concerning the properties of treated sieve material especially as regards adsorption capacity, the selectivity of the treated sieve material for hydrogen halide was found to be excellent.

To test selectivity, a hydrogen stream containing 3.5% HC1 was passed through sieve material, treated in accordance with this invention, at the rate of 1.44 liters per minute. A weight increase of 6% was noted for the sieve. A comparable stream of hydrogen containing 3% trichlorosilane was passed through identically prepared sieve material at substantially the same rate and for the same period and a weight increase of 1.05% was noted for the sieve.

If it is assumed that the entire Weight gain in each case is due to adsorption of HCl or SiHCla, then the mol adsorption factor mols of Sil-1G13 for the data obtained is 21.2 assuming a molecular weight of 36.5 for HCl and 135.4 for SiHCl3.

Applying this data by extension to the reduction of SiHCl3 using hydrogen and passing the reduction products (H2, HC1, and unreduced Sil-1G13) through a sieve prepared in accordance with the present invention, the

excellent selectivity of the prepared sieve for HC1 can be demonstrated. The reaction is described briefly in the foregoing text and may be further illustrated by the following reaction.

If a yield of 25% silicon is assumed, the following data applies per mol of SiHClg. The reduction products issuing from thequartz reactor will contain 0.75'mol of unreacted SiHCl3 (for each mol of SiHCl3 feed), 0.75 mol of HC1 (formation of HC1 at a ratio of 3 mols per 1 (reacting) mol of SiHCl3-0-25 mol SiHCl3 reacted forms 0.75 mol HC1) and H2 which serves as the carrier gas and reactant. Calculating on the basis of complete adsorption of 0.75 mol of HCl, the adsorption of SiHCl3 can be computed using the mol adsorption factor which was previously determined as 21.2.

Mntel m01 of Siuoi3 21 2( mols of HC1 mols of SiHCl3 This means that less than about 4.75% of the SiHCl3 Was adsorbed by the prepared sieve from the reduction products stream.

Although the above data and calculations demonstrate the excellent selectivity of sieves prepared in accordance With the present invention, it will be appreciated that many factors have been ignored, all of which Would make for an even lbetter showing. Such factors as adsorbed Water and surface phenomena account for some of the Weight gain of the sieve material. It is believed that could a careful quantitative study be completed, it would show that the actual percent SiHCl3 adsorbed is substantially less than 4.75% and probably not greater than about l or 2%.

In conclusion, the process of the present invention produced a substantially improved sieve by treating the synthetic zeolite material with an aqueous solution of zinc, calcium or magnesium salt, and in particular a chloride. The concentration (by weight) of the salt can be as dilute or as concentrated as desired. The optimum range of concentration was found to be from about 6% to about 50% by Weight. The soaking time also can vary Widely. The optimum range in this instance was found to be from about six hours to about 500 hours. The washing after soaking is for the purpose of removing excess chloride and, hence, Was usually carried out using distilled Water until it tested negative for chloride ion.

Although certain specific embodiments of the invention have been shown and described, it is obvious that many modifications thereof arepossible. The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the `spirit of the appended claims.

What is claimed is:

1. A method of efiiciently removing hydrogen halide from a process stream consisting essentiallyof hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange using calcium ions, and thereafter passing the process stream through the modified sieve at a temperature from about 30 C. to about 350 C.

2. A method of efi'iciently removing hydrogen halide lfrom a process stream consisting essentially of hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange using zinc ions, and thereafter passing the process stream through the modified sieve at a temperature from about 30 C. to about 350 C. l

3. A method of eiiiciently removing hydrogen halide from4 a process stream consisting essentially of hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange using calcium ions, passing the processstream through the modified sieve at a temperature fromv about 30 C. to aboutV 350 C., and thereafter regenerating the modified molecular sieve for effective repeated use by heating said sieve.

4. A method of ef'liciently removing hydrogen halide from a process stream consisting essentially of hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic zeolite selected from the group consisting of'Type 4A and Type 5A Molecular Sieves by ion exchange using zinc ions, passing the process stream through the modified sieve at a temperature from about 30 C. to about 350 C., and thereafter regenerating the modified molecular sieve for effective repeated use by heating said sieve.V

5. The method of efi'iciently removing hydrogen halide from a process stream consisting essentially of hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic `zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange using calcium ions, passing the processstream through the modified sieve, and thereafter regenerating the modiied molecular sieve by heating said sieve from about 300 C. to about 800 C.

6. The method of efficiently removing hydrogen halide from a process stream consisting essentially of hydrogen halide and a substantially unadsorbable carrier gas, which comprises the steps of modifying a synthetic zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange usingzinc ions, passing the process stream through the modified sieve, and thereafter regenerating the modified molecular sieve by heating said sieve from about 300 C. to about 800 C.

7. A method of efficiently removing the hydrogen halide reaction product resulting from the hydrogen reduction of a halosilane to silicon which comprises the steps of modifying a synthetic zeolite selected from the group consisting of Type 47Arand Type 5A Molecular sieves by ion exchange using calcium ions, passing a heated process stream comprised of halosilane and hydrogen through a reaction zone, removing from the reaction zone a spent process stream comprised of halosilane, hydrogen and hydrogen halide, passing said spent process stream ythrough the modified molecular sieve at a temperature from about 30 C. to about 350 C. to remove selectively hydrogen halide, and thereafter regenerating the modified molecular sieve by heating said molecular sieve from about 300 C. to about 800 C.

S. A method of efficiently removing the hydrogen halide reaction product resulting from the hydrogen reduction of a halosilane to silicon which comprises the steps of modifying a synthetic vzeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves by ion exchange using zinc ions, passing a heated process stream comprised of halosilane 'and hydrogen through a reaction zone, removing from the reaction zone a spent process stream comprised of halosilane, hydrogen and hydrogen halide, passing said spent process stream through the modified molecular sieve at a temperature from about 30 C. to about'350" C. to remove selectively hydrogen halide, and thereafter regenerating the modified molecular sieve by heating said molecular sieve from about 300 C. to about 800 C.

9. A method of efi'iciently removing hydrogen halide' from a process stream consisting essentially of hydrogen halide and a substantially unadsorbablbe carrier gas, which comprises the steps of:

(a) soaking a synthetic zeolite selected from the group consisting of Type 4A and Type 5A Molecular Sieves indan aqueous solution of about 121/2% zinc chlori e;

(b) removing chloride ions from said synthetic zeolite by washing in distilled water;

(c) drying said synthetic zeolite; and

(d) passing the process stream through the synthetic zeolite at a temperature from about 30 C. to about 350 C.

References Cited by the Examiner UNITED STATES PATENTS 1 0 OTHER REFERENCES Sorpton by Gmelinite and Mordente, by R. M. Barrer, Transactions of Faraday Society, volume 40, 1944, pages 555-564.

Barrer article, I. Soc. Chem. Ind., Vol. 64, 5/45, pp. 13G-135.

Breek et al.: article, I. of Am. Chem. Soc., v01. 78, No. 23, Dec 8, 1956, pp. 5963-5971.

REUBEN FRIEDMAN, Primary Examiner. 

1. A METHOD OF EFFICIENTLY REMOVING HYDROGEN HALIDE FROM A PROCESS STREAM CONSISTING ESSENTIALLY OF HYDROGEN HALIDE AND A SUBSTANTIALLY UNADSORBABLE CARRIER GAS, WHICH COMPRISES THE STEPS OF MODIFYING A SYNTHETIC ZEOLITE SELECTED FROM THE GROUP CONSISTING OF TYPE 4A AND TYPE 5A MOLECULAR SIEVES BY ION EXCHANGE USING CALCIUM IONS, AND THEREAFTER PASSING THE PROCESS STREAM THROUGH THE MODIFIED SIEVE AT A TEMPERATURE FROM ABOUT 30*C. TO ABOUT 350*C. 